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The vertical blanking interval (VBI) is the transition in a raster-scanned video signal between the visible picture at the end of one frame or field and the start of the next. In analog television, it allowed the scan to return to the top of the picture and could carry signals or data. It is not necessarily empty of signal, and its line count depends on the video format.
What does “vertical blanking interval” mean?
The International Telecommunication Union (ITU) defines the VBI as the time between the last line of one raster frame or field and the beginning of the first line of the next. It is also called the vertical interval or VBLANK. A raster forms a picture by scanning lines in sequence, so the VBI is a timing interval between visible portions of successive frames or fields—not a separate image. ITU, Making Television Accessible, glossary.
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“Blanking” means the interval is not intended to display the normal picture. It does not mean that every line is electrically silent: synchronization signals and data may be present.
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In analog television, the VBI provided time for the raster scan to return from the bottom of the picture to its beginning before the next frame or field. Because that period was outside the normally displayed picture, broadcasters and video systems could also use it to transmit information.
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Possible uses included closed captions, teletext, time codes, test signals, copy-protection indicators such as CGMS-A, and other data. These are examples of services that could use the interval; they were not all present in every format or broadcast. ITU, Making Television Accessible, glossary.
How many lines are in the VBI?
There is no single line count that applies to every video format. For NTSC, the Advanced Television Systems Committee (ATSC) glossary describes the first 21 horizontal lines in each field as VBI lines not intended for direct display by a receiver. That is an NTSC-specific description, not a universal definition. ATSC, “Vertical blanking interval”.
Is NTSC line 21 part of the VBI?
Its classification depends on the source or convention. NTSC CEA-608 caption data is commonly associated with line 21, but the Federal Agencies Digitization Guidelines Initiative notes that the line sits at the boundary between VBI and active picture: some authorities classify it as part of the VBI, while others treat it as the first active-video line. On underscan displays, scopes, or broadcast measurement equipment, caption data can appear as lines and dashes above the visible picture. Federal Agencies Digitization Guidelines Initiative, “Closed captions — Glossary”.
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SMPTE 170M-2004 provides historical standards context: it describes the portion of the vertical blanking interval outside the nine-line vertical sync block as available for other uses and identifies line 21 data as part of program material. This does not establish a single classification for every NTSC implementation. SMPTE, SMPTE 170M-2004.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does digital video have a VBI?
Digital video does not have the analog VBI as the same interval in a scanned video waveform. However, digital television systems can transport legacy VBI information in a bitstream so equipment can process it or insert it into an output waveform. The distinction is between the original analog timing interval and digital carriage of information associated with that interval.
For example, ANSI/SCTE 127 2019, reaffirmed in 2024, specifies carriage of analog VBI information in compressed digital television bitstreams for output-waveform insertion or direct receiver handling. Its scope excludes CEA-608 Line 21 and CEA-708 caption carriage. ANSI/SCTE 20 2020 separately specifies carriage for CEA-608 captions and certain other VBI services in MPEG-2-compliant streams. SCTE, ANSI/SCTE 127 2019 (R2024); SCTE, ANSI/SCTE 20 2020.
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